This invention relates to the area of switching and transferring data in a mobile communication network. More particularly, this invention refers to a method and a network for interconnecting base stations and mobile switching centers.
The past decade has seen a large deployment of mobile, or cellular, communication systems. Systems such as GSM (Global System for Mobile communication), GPRS (General Packet Radio Service), and CDMA (Code Division Multiple Access) have grown dramatically in coverage, leaving almost no white spots left on the geographical map. With the planned migration and integration into the third generation mobile system UMTS (Universal Mobile Communication System), a remaining obstacle for further increased deployment has been overcome.
With the increased geographical coverage and public usage of such systems follows increased requirements on system capacity and system redundancy. From an operator point of view, requirements on simplicity and efficiency in terms of system design and maintenance become more pronounced.
In a typical prior art mobile communication network, using GSM and GPRS (and UMTS) as an example (see FIG. 1 for reference), the mobile switching center MSC, or its GPRS equivalent SGSN (Serving GPRS Support Node), form the heart of the network. Typically, a transport network is used to transport traffic between the MSC/SGSN and one or more gateways to public switched telecommunication networks and/or to packed data networks, such as an X.25 network or the Internet.
To each MSC/SGSN, a set of base station controllers BSC, (referred to as radio network controllers RNC in UMTS), are connected, and each base station controller is in turn arranged to control a respective set of base transceiver stations, each covering a respective geographical area, or cell, the system in all forming a well defined hierarchical structure.
A problem with such a prior art network is that if a BSC or an MSC goes down, it affects all base stations served by that BSC or MSC, thus disabling mobile access within the affected area. Another problem is that, as a result of the system distribution required to provide for geographical coverage, system maintenance and operation require deployment of manpower and other resources at many different geographical sites. Also, the task of synchronizing the operation of base stations for multi base station transmission requires the distribution of clock information, which for example is complicated in relation to base stations are not controlled by the same BSC/RNC.
It is accordingly an object of the invention to provide a method and a network that provide for simplified mobile communication network maintenance and operation.
It is further an object that the method and network allow for increased redundancy in case of, for example, a BSC or MSC failure.
It is also an object of the invention that it offers a simplified way of providing synchronization of base station operation.
The present invention overcomes the above described problems and deficiencies of the prior art by providing an improved mobile communication method and network by interconnecting mobile switching centers and base station controllers (radio network controllers) and/or base station controllers and base transceiver stations using, at least in part, a Dynamic synchronous Transfer Mode (DTM) type network.
Moreover, according to a preferred embodiment of the invention, the hierarchical relationship between the mobile switching centers, the control stations, and the base stations is controlled by the definition of logical channels within the Dynamic Synchronous Transfer Mode type network.
A Dynamic synchronous transfer mode network provides a multi-access scheme that is used according to the invention to define connectivity on a logical level more or less independently with respect to the underlying physical topology. The invention therefore provides the advantage of significantly increasing the manageability of a mobile communication network.
The DTM network may for example be used to interconnect a mobile service center and the base station controllers (radio network controls) associated therewith, as well as to interconnect a base station controller and the base transceiver stations controlled thereby.
According to an aspect of the invention, data is transmitted between a plurality of mobile switching centers, base station controller and/or base stations over the same physical DTM network, wherein the hierarchical relationship between the different components physically connected to the same network (MSC, BSC, BTS) is defined on a logical level by the establishment of logical TDM channels over the DTM network. Since the need of providing a hierarchical relationship between the components on a physical level is thereby eliminated, the mobile switching center and the base station controllers may in fact be physically positioned at any desired location as long as access is provided via channels within the DTM network. For example, a plurality of MSCs and/or a plurality of BSCs/RNCs may be placed at the same geographical location, thereby advantageously simplifying network maintenance and operation.
Also, in case, for example, a mobile switching center or a base station controller were to fail, another mobile switching center or base station controller can take over the role of controlling the affected area since the hierarchical relationship may, at least to some extent, be re-defined using logical channels within the DTM network and is not limited by the physical connectivity constraints of prior art.
Furthermore, as DTM is a synchronous scheme, operating at a frame rate of nominally 125 xcexcs, the task of providing synchronization in case of multi base station transmissions is simplified by the access to the DTM 125 xcexcs frame clock. This synchronization reference may, for example, be used by the base stations as means for avoiding intersymbol interference when transmitting data.
According to another aspect of the invention, as connectivity may be defined on a logical level within the DTM network, the same DTM network as mentioned above is used to carry data between the mobile switching centers (MSC or SGSN) and the gateway to the Public Switched Telecommunication Network (PSTN) and/or to a packet switching network, such as the Internet. Thus, according to the invention, instead of, for example, having to operate two different networks as in prior art, one used between the gateways and the MSCs and one between the MSCs and the BSCs, only one physical network needs to be managed, everything else being handled within that network on a logical level.
Also, with the use of DTM for providing connectivity according to the invention, bandwidth resources may at each time be logically allocated to those MSCs, BSCs or BTSs where they are needed the most.
For general information on DTM networks and on how DTM channels are established in a DTM network, reference is made to xe2x80x9cThe DTM Gigabit Networkxe2x80x9d, Christer Bohm, Per Lindgren, Lars Ramfelt, and Peter Sjodin, Journal of High Speed Networks, Vol. 3, No. 2, pp. 109-126, 1994, and to ongoing standardization processes within the European Telecommunication Standards Institute (ETSI).
However, in brief, a dynamic synchronous Transfer Mode type network is a network having a dynamic frame structure in that the bandwidth of the network links are divided into recurrent, essentially fixed sized frames which in turn are divided into fixed size time slots, wherein write access to the time slot positions within the recurrent frame of a link is arbitrarily distributed to the nodes connected to the link to define logical channels on said link, said channels being defined by respective one or more time slot positions within said recurrent frame of the link. Consequently, the size of a channel will depend upon the number of time slots allocated to the channel. Moreover, the frame structure is dynamic in that the said distribution of write access to the time slot positions of a link may at any time be redefined, i.e. causing a redistribution of time slot access among the nodes attached to the link and/or among the channels served by the respective nodes. Also, in a Dynamic synchronous Transfer Mode type network, a channel may be defined to extend over several links (a link typically being a bus, double-bus, ring, double-ring, or a point-to-point connection), wherein the channel will be defined by a respective set of one or more time slot positions within the recurrent frames of the respective links that said channel is established over.
To be noted, even though the invention has been described primarily with reference to GSM, UMTS and GPRS systems, the invention shall of course not be considered limited thereto, the scope of the invention also encompassing other mobile networking standards and architectures of similar general design and structure.